Soon after the introduction of Frozen Concentrated Orange Juice (FCOJ) in 1948, it was discovered that the FCOJ gelled when left at room temperature. Early FCOJ was concentrated above the desired Brix on low temperature evaporators and "cut back" juice was normally added to reduce the Brix to the desired level and to add flavor. "Brix" as used in citrus processing generally refers to the percent by weight of sugars in the citrus juice value obtained by a refractometer, corrected for temperature, to which is added a correction for citric acid.
Gelation is highly undesirable because it results in a high viscosity citrus concentrate product. A high viscosity product leads to problems in juice processing, evaporation, pumping, and storage of citrus concentrates. High viscosity reduces the heat transfer rates and flow of the falling concentrate film on the evaporation tubes and may cause the concentrate to "burn on" the hot surfaces, resulting in loss of product quality. In addition, a high viscosity concentrate product is more difficult to pump and can cause clogging is post-mix juice dispensers.
There are essentially two types of gels that can be formed with fruit juice pectins: low methoxyl (LM) and high methoxy (HM). The LM gel occurs when a pectin with a degree of esterification (DE) of less than 50% is combined with divalent ions to cross link the pectin structure. Orange juice has an abundance of divalent calcium ions (about 100 mg/Kg in 11.8.degree. Brix juice) naturally present to do this cross linking. It is believed that the pectin chains are aligned so that parallel chains provide a cup or cavity of oxygen atoms for the Ca.sup.++ ions. A series of calcium atoms can fit between the pectin chains forming an "egg box" shaped arrangement. This type of gel is mechanically and thermally reversible; it can be liquified by shearing (e.g., by mechanical blending) or by heating, but the gel reforms spontaneously after shearing is stopped or heating subsides. Commercially, this type of gel is used primarily in milk-type systems to produce such items as snacks puddings.
The second type of gel, HM, is also known as the sugar-acid type of gel. HM gels, in turn, can be divided into two broad types; rapid set (which sets in about 20 to 70 seconds and at a high temperature); and slow set (which sets in about 180 to 250 seconds and at a lower temperature). Commercially, HM type gels are used to make jams and jellies.
It is believed that the HM gel structure has junction zones of pectin molecules stabilized by both hydrogen bonding and hydrophobic forces. The hydrogen bonds are twice as strong as the hydrophobic forces, but require the addition of the hydrophobic forces in order to stabilize the gel. The hydrophobic forces are, in turn, stabilized by a high concentration of sugar at the proper pH. Unlike the LM gel, this type of gel does not require divalent ions for formation.
During the 1950's, the gelation problem became so severe that it threatened the infant citrus processing industry with extinction. Gelation of citrus concentrate was found to be caused by the naturally occurring enzyme, pectinesterase (PE). This enzyme was able to produce gelatin when pasteurization was delayed or the pasteurization temperature was insufficient to inactivate the native PE. PE was found to remove the methoxyl groups from the pectin and it was believed that this made the pectin more sensitive to cross linking by naturally occurring calcium ions in the juice. Together, the demethoxylated pectin and calcium, under the conditions in the concentrate, formed a low methoxyl (LM) type of gel. Later, it was discovered that when the citrus juice was rapidly heated to a high enough temperature, PE was inactivated before it could demethoxylate the pectin such that LM gelation did not occur. The replacement of the low temperature evaporators by Thermally Accelerated Short-Time Evaporators (TASTE) in the 1960's solved most citrus juice gelation problems by rapidly and completely inactivating PE.
Traditionally, 41.8.degree. Brix orange concentrate is stored at temperature ranging from about -11.degree. C. to 0.degree. C. It has been estimated that increasing the concentration and storing or transporting the orange concentrate at refrigerated temperatures of 3.degree. C. to 6.degree. C. could result in savings of about 15% in storage costs and about 30% in refrigeration costs. These savings are particularly important in light of the 1.times.10.sup.6 tons of bulk orange concentrate that is shipped in the annual world market.
Unfortunately, to date is has not been practical to increase the concentration of orange concentrate much beyond about 62.degree. Bris to 65.degree. Bris because the resulting increased concentration of sugar causes extremely high viscosity and HM gelation. These high viscosities prevent the use of such high Brix concentrations with existing processing and storage equipment. Because it is economically advantageous to go to such higher Brix concentrations, there is a need to develop a method for preventing HM gel formation with its accompanying increase in viscosity. The present invention provides a method for achieving this objective.